Yes, she cheated on Joachim with Martin while still married.
IMO, what Alex is experiencing right now is the classic If he/she cheats WITH you, he'll likely end up cheating ON you.
I honestly never believed that Martin loved her. At least not as much as she loved him.
She married up the first time - Jokke opened the door to an initially very exciting new life as a European princess. She played the role very well. I don't think she loved Jokke as much as she loved the position he enabled her to have. Without Jokke she would have been one of these upper middle class career women, I think. The life Jokke gave her was way more exciting.
Now as you say she is at the other side of the fence. She opened the door to Martin to an inititally very exciting new life (diplomatic status, loads of money, different lifestyle). And she liked her role as the hipster mom with the young lover/ husband. Sadly she had to find out that Martin loved the position she enabled him to have more than her. Without her ... he would have been nothing. Not even an upper middle class businessmen.
Back then she had little consideration for Jokke and she moved on very quickly after the divorce. Bad headlines for her meant bad headlines for the DRF. Now she is in the same situation.